Method of determining type of plastics

Abstract: The method of the present invention comprises the steps of irradiating near-infrated rays to a plastics of an unknown type to measure the NIR spectrum thereof, determining the first-derivative or second-derivative, measuring the differential spectrum at prescribed intervals of wavelength over a region of measurement wavelengths, determining the class of whether the differential spectrum for each measurement wavelength is on the plus-side, minus-side or any other side, comparing the resultant class data with those of previously determined differential spectra for plastics of known types to determine the ratio of agreement between the thus compared data (agreement ratio), and determining that the plastics showing the highest agreement ratio is the type of the plastics of the unknown type. By using the above-mentioned agreement ratio, it is possible to simply and rapidly determine the type of a plastics by means of the differential spectrum of near-infrared rays.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of determining the type of plastics. In particular, but not exclusively, the present invention relates to a method of determining the type of plastics, which, in disposing of or recycling diverse waste of plastics such as plastic bottles for soft drink and plastic packages of foods, is adapted to be used for classifying plastic waste by materials. Various types of plastics including plastic bottles for soft drink and plastic packages of foods are rejected as waste after use in large quantities and are disposed of by incineration or dumping for reclamation. A problem is however that incineration is not suitable for some types of plastics because the incinerator is damaged, and it is becoming more and more difficult to find new incinerating yards and land for reclamation. It is therefore tried now to manufacture petroleum and other fuel from plastic waste, or to recycle waste by bringing back the original plastic materials for each type of plastics. Under such circumstances, it is becoming more important than ever to determine the type of plastics and classify plastic waste into individual types. The conventional methods already in industrial use for classifying plastics include a method using the difference in specific gravity between different types of plastics and a method of identifying polyvinyl chloride based on X ray examination. In the method based on specific gravity, however, distinguishing between polyethylene and polypropylene and between polyvinyl chloride and polyethylene terephthalate, is difficult because of the relatively slight difference in specific gravity. Another problem is that specific gravity of plastics varies as a result of surface wetting of plastics or kneading with pigment or filler, so that these methods are far from being satisfactory. In the method based on X rays, use of radiation poses problems in safety and management, and in addition, polyvinyl chloride is the only plastics that this method can identify and classify, so that this cannot be practicable over a range of plastics. JOURNAL OF APPLIED POLYMER SCIENCE, Volume 31, No. 5, April 1986, LG Weyer "Utilising zero crossover points in the near infrared reflectance analysis of industrial polymers", pages 2417-2431, discloses near infrared reflectance methods developed for the analysis of polymer pellets and films. Second derivative spectra were used to minimize the effects of scattering and to enhance spectral features. Zero crossover points, places in the derivative spectra where there is negligible response to concentration changes, were employed to minimize interferences at the measuring wavelengths. Two examples illustrate the utility of this technique for industrial quality assurance analyses. The examples include the measurement of three hydrocarbon polymers, directly in transparent pellets, and the determination of 0.1% MEK in an acrylic film. The present invention is as claimed in the claims. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Fig. 1 is a graph illustrating NIR(near infrared) spectra of polyethylene used for the preparation of a reference file in an embodiment of the method of determination of the present invention; Fig. 2 is a graph illustrating NIR spectra of polypropylene used for the same purpose as in Fig. 1; Fig. 3 is a graph illustrating NIR spectra of polyvinyl chloride used for the same purpose as in Fig. 1; Fig. 4 is a graph illustrating NIR spectra of polyethylene terephthalate used for the same purpose as in Fig. 1; Fig. 5 is a graph illustrating NIR spectra of polystyrene used for the same purpose as in Fig. 1; Fig. 6 is a graph illustrating first-derivative NIR spectra of polyethylene shown in Fig. 1; Fig. 7 is a graph illustrating first-derivative NIR spectra of polypropylene shown in Fig. 2; Fig. 8 is a graph illustrating first-derivative NIR spectra of polyvinyl chloride shown in Fig. 3; Fig. 9 is a graph illustrating first-derivative NIR spectra of polyethylene terephthalate shown in Fig. 4; Fig. 10 is a graph illustrating first-derivative NIR spectra of polystyrene shown in Fig. 5; Fig. 11 is a graph illustrating second-derivative NIR spectra of polyethylene shown in Fig. 1; Fig. 12 is a graph illustrating second-derivative NIR spectra of polypropylene shown in Fig. 2; Fig. 13 is a graph illustrating second-derivative NIR spectra of polyvinyl chloride shown in Fig. 3; Fig. 14 is a graph illustrating second-derivative NIR spectra of polyethylene terephthalate shown in Fig. 4; Fig. 15 is a graph illustrating second-derivative NIR spectra of polystyrene shown in Fig. 5; and Fig. 16 is a descriptive view illustrating setting of standard region of determination of setting of standard region of determination of class at individual measurement wavelengths on derivative spectra in the method of a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 1 to 5 are graphs illustrating NIR spectra of plastics used for the preparation of reference files in an embodiment of the method of determination of the present invention. The NIR spectra were measured from a transmitted beam of near-infrared rays. Fig. 1 shows NIR spectra obtained by irradiating with near-infrared rays while continuously scanning within a wavelength region of from 1 to 2.5 µm for seven kinds of polyethylene (PE). Similarly, Fig. 2 shows NIR spectra determined for five kinds of polypropylene (PP); Fig. 3, three kinds of polyvinyl chloride (PVC); Fig.4, three kinds of polyethylene terephthalate (PET); and Fig. 5, two kinds of polystyrene (PS). There are present, in the near-infrared region, overtones and combination tones of absorbance of the infrared region, which provide much information for qualitative and quantitative analyses of moisture and organic substances. Abundance of information however forms noise signals in the analysis of a target constituent and this often hinders smooth analysis. In spectroscopic analysis in the near-infrared region, therefore, a statistical technique such as multivariate analysis is employed to derive information which meets the purpose from the available NIR spectra. That is, analysis of NIR spectra is now an important element in the spectroscopic analysis in the near-infrared region. In the present invention a method of determining the type of plastics through a simple spectral analysis is achieved while avoiding the adoption of such a complicated statistical technique. More specifically, the method may comprise measuring the NIR spectrum of the target plastics, calculating a derivative spectrum, and determining the type of plastics from whether the wavelength region containing the peak of the resultant derivative spectrum (differential peak) agrees or not. The technique of determining a derivative spectrum such as a first-derivative or a second-derivative through measurement of NIR spectrum is a known one, and permits expectation of such effects as: (1) eliminating biases present over the entire region of measurement, such as the influence of opacity or stain of the sample of measurement; (2) converting superposed spectra into signals which are easier to analyze; and (3) the presence of a zero-crossover point on the wavelength axis. First-derivatives derived from the NIR spectra shown in Figs. 1 to 5 are illustrated in Figs. 6 to 10, and second-derivatives thereof, in Figs. 11 to 15. In the first-derivatives and the second-derivatives, biases present throughout the entire measurement wavelength region as observed in the NIR spectra have been eliminated, and superposed peaks have been converted into a waveform easier to analyze. A zero-crossover point is existent on the wavelength axis. This zero-crossover point is the point where the waveform curve of the NIR spectrum intersects the wavelength axis having a differential value of zero. In the first-derivative, this corresponds to the maximum peak wavelength of NIR spectrum, and in the second-derivative, to the wavelength at which a point of inflection of NIR spectrum appears. The zero-crossover point in a derivative spectrum is therefore a singular point of function intrinsic to a particular type of plastics, irrespective of the thickness of density of the particular plastics. It is estimated from this fact that when this zero-crossover point differs in position between two plastics, i.e., if the wavelength regions are different of the peak of derivative spectrum (differential peak) appearing between the zero-crossover points, the two plastics are of different types, and if identical on the other hand, these must be plastics of the same type. As is clear from Figs. 6 to 10 and 11 to 15, the wave-length region in which the differential peak appears differs with the type of plastics for both first-derivatives and second-derivatives. It is therefore possible to determine the type of an unknown plastics by previously determining the wavelength region in which the differential peak appears for a plastics of a known type, and seeing whether or not the wavelength region in which the differential peak appears of an unknown plastics agrees with it. The method of a first embodiment of the present invention comprises the following steps. In the description hereafter, data processing is accomplished by a computer connected to a near-infrared spectrometer. (1) First, measuring continuously the near-infrared absorption spectrum in a prescribed wavelength region such as from 1 to 2.5 pm for a plastics of a known type, determining first-derivative or second-derivative thereof, and determining the derivative spectrum at prescribed intervals of wavelength within a range of from several nm to 10 nm, at intervals of 6 nm for example, over the entire measurement wavelength region, thereby determining the class as to whether the derivative spectrum for each of the measurement wavelengths is on the plus-side, minus-side or any other side. This is applied to some plastics of the same category, such as seven kinds of polyethylene, and classes of the individual derivative spectra are labelled with signs "+1," "-1" or "0," depending upon whether that class of derivative spectra is on the plus-side, minus-side (ie positive or negative in value) or on any other side. Similarly, the same steps are followed also for other types of plastics. The "any other side" could include plus-side or minus-side values of absolute value less than a predetermined threshold value, for example. (2) Then, preparing a classification table showing classification data of derivative spectra with corresponding numbers of measurement wavelengths as shown in Table 1, and storing the prepared table in the memory as a reference file for each type of plastics. Table 1 shows 256 data obtained by measuring a measurement wavelength region of 1 to 2.5 µm at intervals of wavelength width of 6 nm. Data No. 0 1 2 3 ... n-1 n n+1 ... 253 254 255 Label 0 0 +1 +1 -1 -1 0 0 0 0 (3) Conducting the same operation also for plastics of an unknown type, continuously measuring NIR spectrum to determine the derivative spectrum, determining the derivative spectrum at equal intervals of wavelength over the range of measurement wavelengths, determining the class as to whether the derivative spectrum of each measurement wavelength is on plus-side, minus-side, or on any other side to label "+1," "-1" and "0," and preparing a classification table showing classification data of derivative spectra with corresponding data numbers. It is assumed that a classification table as shown in Table 2 is obtained. Data No. 0 1 2 3 ... n-1 n n+1 ... 253 254 255 Label 0 0 +1 +1 0 -1 -1 0 0 0 (4) Then, comparing all the data of the classification table of an unknown plastics with all the data of the classification table of reference file for each data No. one by one to determine the number of data showing agreement of labels. Conducting this operation for all the reference files of the previously stored known plastics. (5) Then, determining the ratio of the number of data showing labels in agreement to all the number of data, investigating the highest agreement ratio, and determining that the plastics of the reference file of the highest agreement ratio is the type of the unknown plastics. The use of the above-mentioned agreement ratio means determination of the degree of agreement of the wavelength region in which the differential peak of the unknown plastics appears with the wavelength region in which the differential peak of the known plastics appears. The plastics showing the highest agreement ratio is the type of the unknown plastics, thus the type of the unknown plastics is determined. This of course assumes that the classification table of all plastics which may appear as the unknown plastics are available. According to the above-mentioned embodiment of the method of the present invention, therefore, it is possible to simply and rapidly determine the type of an unknown plastics. The second embodiment of the present invention, which permits further improvement of the determination accuracy and is therefore more preferable, comprises the following steps; (1) First, for the purpose of increasing the determining accuracy of the class of first-derivative or second-derivative for each measurement wavelength, determining the maximum value (assumed to be Px) of the absolute value of derivative spectrum in the measurement wavelength region, as shown in Fig. 16, and setting a determination standard region marked off by certain plus and minus values selected with the maximum value to standardize the derivative spectrum, for example, taking 10% of the maximum value: A = 0.1 Px and B = -0.1 Px (a region surrounded by lines parallel to the wavelength axis passing through values A and B of the derivative in Fig. 16). (2) Then, labelling the class of the differential spectrum for each measurement wavelength as "+1," "-1" or "0," respectively, depending upon whether the derivative spectrum is over, under or within the determination standard region, preparing a classification table containing class data of the derivative spectrum with corresponding data number, and storing the table as the reference file for each class of plastics. (3) Conducting the same operation for the plastics of an unknown type, labelling the class of the derivative spectrum for each measurement wavelength as "+1," "-1" or "0," respectively, depending upon whether the derivative spectrum is over, under or within the determination standard region, and preparing a classification table containing class data of the derivative spectrum with corresponding data numbers. (4) Then, comparing data of the classification table of the unknown plastics with data of the classification table of reference file for each data No. one by one to determine the number of data showing lables in agreement - such comparison is not made, however, for data numbers labelled "0" in the reference file - conducting this operation for all the reference files of the previously stored known plastics. (5) Then, determining the ratio of the number of data showing labels in agreement to the total number of data (naturally excluding the number of data labelled "0"), investigating the highest agreement ratio, and determining that the plastics of the reference file of the highest agreement ratio is the type of the unknown plastics. According to this second embodiment, the class of derivative spectrum for each measurement wavelength is determined by setting the determination standard region marked off with certain plus and minus values with the maximum absolute value of derivative spectrum to standardize the derivative spectrum, while avoiding comparison of data labelled "0" which have a small S/N ratio of waveform (differential signal) because of the slight value of derivative spectrum (differential value), thus permitting remarkable improvement of the determining accuracy of the class of plastics. Both the above-mentioned embodiment have comprised continuously measuring NIR spectrum within a prescribed wavelength region to determine the derivative spectrum thereof, and measuring the derivative spectrum at prescribed intervals of wavelength over a region of measurement wavelengths although small discrete steps of wavelength can be used provided the required derivatives can be obtained thereby determining the class of the derivative spectra at each measurement wavelength. The present invention is not however limited to these embodiments, but the kind of derivative spectrum at each measurement wavelength may be determined, by measuring NIR spectrum at prescribed intervals of wavelength from the very beginning over a prescribed region of wavelengths, from derivative spectra thereof. In the above-mentioned embodiments, a classification table of derivative spectra at each measurement wavelength has been prepared for an unknown plastics, and data of this classification table have been compared with data of a classification table of reference file. It is not always necessary, however, to compare the class data of derivative spectra of the unknown plastics after preparing the classification table, but comparison with the data of the classification table of reference file may be made every time a class data of the differential spectrum at each measurement wavelength is obtained. A first-derivative or second-derivative has been used in the above-mentioned embodiments, but a similar operation may be accomplished through determination of a tertiary or a higher-degree derivative spectrum. The absorption spectrum can be measured from a transmitted beam or reflected beam of near-infrared rays irradiated to the target plastics. EXAMPLE Now, the present invention will be described further in detail by means of an example. In accordance with the present invention, NIR spectrum was continuously measured in a wavelength region of from 1 to 2.5 µm for plastics samples including PE ( polyethylene), PET (polyethylene terephthalate), PP (polypropyrene), PVC (polyvinyl chloride) and PS (polystylene) in the form of plastics bags and the like. Second derivatives were determined from the results of measurement, and the derivative spectrum was measured at intervals of wavelength of 6nm. With the use of the method of the second embodiment, the class of the derivative spectrum for each measurement wavelength was determined. The labels of these class data were compared with the lables of all the data numbers in the classification table of reference file previously prepared for secondary derivative spectra shown in Figs. 11 to 15, except for the data numbers labelled "0," to determined the type of plastics samples from the agreement ratios. The types, agreement ratios and the results of determination for these plastics samples are shown in Table 3. As shown in Table 3, each of the plastics samples shows a maximum agreement ratio of at least 70% at a point corresponding to the type of plastics employed, thus demonstrating that the type of plastics was accurately determined from the agreement ratio. As described above, the method of the present invention permits the simple and rapid determination of the type of the unknown plastics. The method of the present invention is suitably applicable for the determination of the type of plastics in various areas, including classification of plastics waste into individual materials.

Claims: A method of determining a type of an unknown plastic, comprising the steps of: (a) irradiating the unknown plastic with near-infrared rays to measure an absorption spectrum of the unknown plastic over a measurement wavelength range within a near infrared-region; (b) calculating and measuring a derivative spectrum of the unknown plastic at equal intervals of wavelength of up to 10 nm to provide derivative spectrum measurement for each measurement interval; (c) determining a class of the derivative spectrum measurement for each interval; (d) comparing the class of the derivative spectrum measurement of the unknown plastic with a class of a derivative spectrum measurement previously determined for each of a plurality of plastics of known types for each interval; (e) calculating a ratio of agreement for each known plastic by dividing the number of times the class of the derivative spectrum measurement of the unknown plastic agrees with the class of the derivative spectrum measurement of each known plastic by the total number of intervals compared; and (f) determining the type of the unknown plastic as the known plastic having the highest ratio of agreement; the step of determining a class of the derivative spectrum measurement for each interval comprising either: (g) determining whether each derivative spectrum measurement is positive, negative or zero; or (h) determining a maximum absolute value of the derivative spectrum measurements within the measurement wavelength range to provide a standard determination region defined within upper and lower boundaries, respectively above and below the wavelength axis, at a distance from the wavelength axis equal to a predetermined fraction of the maximum absolute value and determining whether each derivative spectrum measurement is over, under, or within the standard determination region. The method of determining the type of an unknown plastic as claimed in claim 1, wherein said plastics of known types include polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polystyrene. The method of determining the type of an unknown plastic as claimed in claim 1 or 2, wherein said derivative spectrum is a first-derivative or a second derivative. The method of determining the type of an unknown plastic as claimed in claim 1, 2 or 3 wherein said spectrum is measured for a transmitted beam of the near-infrared rays irradiated onto said unknown plastic. The method of determing the type of an unknown plastic as claimed in claim 1, 2 or 3, wherein said spectrum is measured for a reflected beam of the near-infrared rays irradiated on said unknown plastic.

Inventors: AIKAWA KATSUAKI, SHIRATAMA TADASHI, AIKAWA, KATSUAKI, SHIRATAMA, TADASHI, AIKAWA, KATSUAKI, C/O TOA ELECTRONICS LTD., SHIRATAMA, TADASHI, C/O TOA ELECTRONICS LTD.
Applicants: TOA ELECTRONICS LIMITED
Application year: 1994
Main classification: G01J3
IPC codes: G01N33, G01N21, G01J3